Cameron said of Mantel: "I think she writes great books, but I think what she’s said about Kate Middleton is completely misguided and completely wrong.

"What I’ve seen of Princess Kate at public events, at the Olympics and elsewhere is this is someone who’s bright, who’s engaging, who’s a fantastic ambassador for Britain. We should be proud of that, rather than make these rather misguided remarks."

Mantel said in a speech that the duchess appeared to have been "designed by a committee and built by craftsmen, with a perfect plastic smile and the spindles of her limbs hand-turned and gloss-varnished". The speech, titled Royal Bodies, was given at a London Review of Books event at the British Museum on 4 February.

Newspaper reports interpreted the remarks as a direct attack on the duchess. The Daily Mail called it a "venomous attack on Kate", and the Sun said it was a "bizarre rant".

Mantel was not commenting on the backlash, but a spokesman for the author urged people to read or listen to the speech "because it puts everything in its full context".

A good chunk of the speech focused on how the media reported on the monarchy and their every movement. Mantel reflected that when the duchess’s pregnancy became known, the duchess had been filmed running a few paces with a hockey stick.

"BBC News devoted a discussion to whether a pregnant woman could safely put on a turn of speed while wearing high heels," Mantel said. "It is sad to think that intelligent people could devote themselves to this topic with earnest furrowings of the brow, but that’s what discourse about royals comes to: a compulsion to comment, a discourse empty of content, mouthed rather than spoken."

She compared the monarchy to pandas: "Our current royal family doesn’t have the difficulties in breeding that pandas do, but pandas and royal persons alike are expensive to conserve and ill-adapted to any modern environment. But aren’t they interesting? Aren’t they nice to look at?"

On Tuesday the duchess made her first public appearance since she presented the BBC sports personality of the year award in December. She visited the Hope House addiction treatment centre in south London.

Cameron’s intervention may raise eyebrows after he referred to the duchess as "Princess Kate". Since her marriage to Prince William she has preferred to be known as Catherine and is officially known as a duchess rather than a princess.

What a sad person Hilary Mantel must be Boo Hoo, still angry over Mrs Thatchter even though the poor woman is dead. she should join Vivienne Westwood who is successful but full of spite too. go live on an island, give us all a break !!!